Currently, in representative motor driving systems using a DC power supply, that have been commercialized or proposed, there are (1) those comprising a PWM inverter and a three-phase synchronous motor, (2) those comprising a constant current power supply, a multi-phase constant current inverter and a multi-phase constant current motor, (3) those comprising a PWM inverter and a three-phase induction motor, and (4) those comprising a switched reluctance motor (e.g. Patent Document 1).
The motor driving system (1) has existed for many years, and has advanced rapidly in the last few years by using a strong neodymium magnet in the motor. The motor driving system (2) was invented by the same inventor as the present invention, and makes it possible to reduce the size and weight and increase the efficiency of the motor as compared with the motor driving system (1) when using a neodymium magnet instead of an electromagnet. Furthermore, the motor driving system (2) enables regenerative braking until stop, making it possible to improve energy recovery efficiency.
The motor driving system (3) has existed for many years, and its commercialization was promoted in Japan in advance of the world. The motor driving system (3) does not require detection of the angular position of a rotor in the motor, so that its structural simplification and good controllability are appreciated. Today, it is widely used in trains, elevators and so on. However, in the motor driving system (3), a three-phase power supplied to the three-phase induction motor is required to be a complete sine wave. On the other hand, the pseudo sine wave output from a PWM inverter has many harmonic components, which is a cause of resisting torque. This significantly reduces the efficiency of the three-phase induction motor. Furthermore, although regenerative braking is theoretically possible, it is necessary to separately provide an inverter for regeneration, resulting in a complex structure.
The motor driving system (4) has been proposed for many years, and has been studied by many researchers. However, it lacks practical use at present because there is no basis for its superiority to the motor driving system (3) described above, and particularly because there is no specific method of electric braking.
[Patent Document 1: Japanese Laid-open Patent Publication 2008-125195]